Oxides of certain transition metals in their highest oxidation state exhibit the useful property that they are capable of acting as host materials to receive guest atoms such as hydrogen and alkali metal atoms. These may be reversibly accepted and given up by the oxide when made a component of a suitable electrochemical cell. The oxides undergo a change in light absorbency generally resulting in a visible colour change as the concentration of guest atoms therein varies. The best known examples of such materials are the tungsten bronzes in which the host material is tungsten oxide (WO.sub.3). Tungsten bronzes are capable of reversibly receiving guest hydrogen or alkali metal atoms and exhibit a colour change in so doing. In the case of tungsten bronzes, the colour change of thin layers in transmitted light is from colourless to a blue colour of an intensity depending upon the concentration of the guest metal atoms. The introduction and removal of the guest atoms to and from the host oxide material may conveniently be achieved by setting up an electrochemical cell in which there is an electrode in connection with the oxide bronze, and electrolyte, preferably a solid electrolyte, a sink and source material for the guest atoms and a second electrode in connection with the sink and source materials. The material acting as a sink and source for guest atoms can conveniently be a second portion of oxide bronze material. The various components of the electrochemical cell can be conveniently be made in the form of thin layers deposited on a substrate.
Devices of this kind are known for use as display devices. They may also be employed to make variable transmission optical devices such as variable optical density windows. In this connection, the term "windows" is not restricted to windows for buildings although such windows are included. Rather, the term refers to any optical device through which light is to be transmitted.
If in such devices the guest atoms are hydrogen atoms, they can be derived from electrolysis of small quantities of water absorbed in the device. However, this presents substantial disadvantages in terms of the durability of the device.
Where the guest atoms are metal atoms, a difficulty arises in making a variable transmission optical device in that although the guest metal atoms may conveniently be transported from one layer to another of an eletrochemical cell, they would normally produce colouration in whichever oxide bronze layer of such a device they may for the moment occupy and known suitable source and sink materials other than oxide bronzes do not form colourless transparent layers suitable for use in devices of this kind.
In known dilute oxide bronzes, the colouration seen in transmission when a concentration of guest metal atoms is present in the oxide bronze is due to a light absorption band which peaks in the infra red but extends to some extent into the red end of the visible spectrum, thus producing a blue colouration effect.